Ductility of Zircaloy-4 Fuel Cladding and Guide Tubes at High Fluences

摘要

Zircaloy fuel cladding suffers progressive degradation of ductility as its neutron exposure and hydrogen uptake increase with burnup. The loss of ductility appears to be the key property governing the cladding integrity in service. We report ductility data of Zircaloy-4 fabricated in stress-relief annealed (SRA) and recrystallized (RXA) conditions, covering a range of fluence, hydrogen content, and irradiation and test temperature. The testing was performed on unirradiated and irradiated Zircaloy-4 cladding and guide tubes in SRA and RXA conditions, respectively, in the temperature range 25-350°C. These materials had been exposed to an estimated neutron fluence of ～8 to 10 x 10~(25) n/m~2 (E > 1 MeV) over four cycles of PWR operation. Due to its RXA fabrication condition, the guide tube material was also believed to represent BWR cladding. The hydrogen contents in the irradiated cladding was in the range of ～200-600 ppm, exhibiting typical radial distribution of circumferential hydrides. By comparison, the hydrogen content in the irradiated guide tube material was in the range of ～250-1800 ppm, exhibiting fairly uniform through-wall distribution of circumferential hydrides. Tensile tests and hydraulic burst tests were conducted on 130-150 mm long tubular specimens. In addition, smaller specimens machined in the form of 55 mm long curvilinear dog-bone and 10 mm slotted semicircular arc were tested in plane stress and plane strain configurations. Corresponding unirradiated archive materials in as-received condition and with uniform hydrogen charging up to 1200 ppm were also tested by identical methods. In all tests the fracture mode was examined by SEM fractography. The investigations revealed a decrease in ductility of Zircaloy-4, mainly caused by irradiation and only partly by increasing hydrogen content. Unlike the elongation data, the strength data remained nearly constant with increasing hydrogen content in both materials at all test temperatures. The irradiated RXA material showed better ductility than irradiated SRA material at equivalent hydrogen levels, and exhibited a clearer correlation of ductility with hydrogen content, mainly due to its uniform hydrogen distribution. The paper will provide these and other quantitative data, e.g., those correlating ductility with the local (near fracture surface) hydrogen content. The paper synthesizes the experimental results and discusses their possible application to the criteria for hydrogen concentration and ductility limits in high burnup fuel.